This invention relates generally to the field of optical data transmission and more specifically relates to measuring the peak-to-peak optical power incident on a photodetector connected to a transimpedance amplifier having an in situ optical power meter function.
Increasingly so, today""s communications uses optical data transmitted through, for instance, a fiber optic cable. At the receiving end of a fiber optic link, a photodetector receives the light and generates an electrical current proportional to the intensity or power of the light. The photodetector can be for short wavelength and long wavelength light sources. This photocurrent is then conditioned and coupled to a transimpedance amplifier. A transimpedance amplifier is an electronic circuit which converts an input signal current into a proportionally scaled output voltage signal. The output of the transimpedance amplifier can be input into a host such as a data processing system, such as a computer. A photoreceiver, comprised of a photodetector and a transimpedance amplifier, can be packaged into a TO can. A TO can is a small, hermetic cylindrical package having a window or a lens on one end to couple the incoming optical data onto a photodetector. The photodetector converts the light to a current which is input to a transimpedance amplifier, and other electronics. On the other end of the TO can are electrical contact pins to transmit electrical data output derived from the optical input and power and ground pins. Because of convention, size and other limitations, the vast majority of TO cans are constrained to, at most, four pins.
An optical power meter is a device which converts light power to a measurable current or voltage that is proportional to the optical input. Optical power detectors can be quite elaborate and expensive. The optical power meter function may be used to monitor the power of the laser generating the optical signal, to measure the loss through the transmission medium, to test the receiving electronics, etc. Typically, to monitor the optical power, the optical fiber is detached from the photodetector associated with the transceiver and the impinging light is attached directly to a separate optical power meter. Then to use the link to receive data again, the fiber is reattached to the optical fiber link. The four-pinned version of the TO can does not have an optical power meter because all four pins are utilized for power, ground, and signaling. Some optical links don""t measure the optical power at all, but rather use a xe2x80x9closs of signalxe2x80x9d detector which indicates when light is not being received or the photodetector is not working.
There is a need in the optical transmission industry to monitor the optical power received by a link in situ to detect if the laser is losing power which might indicate that the laser or the link may need replacement, or to detect if the link is otherwise faulty.
There is a further need in the industry for a low cost optical power meter function which can be implemented in a TO can or other fiber link package without either introducing more pins or without removing an existing pin function.
Other objects, features, and characteristics of the invention; methods, operation, and functions of the related elements of the structure; combination of parts; and economies of manufacture will become apparent from the following detailed description of the preferred embodiments and accompanying figures, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures.
To satisfy the above objects and to provide the industry with a solution to the problems stated above, what is presented herein is an optical receiver comprising a photodetector current source having as output a peak-to-peak current proportional to light impinging on the photodetector, and a peak detector circuit having as input the peak-to-peak current to create a peak voltage that is related to the peak optical power of the impinging light. The peak voltage is in a known relationship to the peak optical power of the impinging light. The optical receiver may further comprise at least one amplifying circuit to generate a peak-to-peak voltage signal from the peak-to-peak current; the peak voltage determined by detecting the peak-to-peak voltage signal; a peak common mode control circuit having as input the peak voltage, the peak common mode control circuit configured as a current sink; and a differential amplifier stage wherein the peak-to-peak voltage signal is imposed on the peak voltage using the current sink.
In a preferred embodiment, the peak detector circuit, the at least one amplifying circuit, the peak common mode control circuit, and the differential amplifier stage are in a transimpedance amplifier connected to the photodetector. The transimpedance amplifier and the photodetector may be packaged in a fiber optic transceiver. The fiber optical transceiver may further comprise a post amplifier connected to the transimpedance amplifier to receive and extract the peak-to-peak voltage signal and generate an optical power signal from the peak voltage, and to interface the peak-to-peak voltage signal and the optical power signal to a host; and a phototransmitter to receive electrical signals from the host and in response thereto generate modulated optical data from transmission. The fiber optical transceiver may be packaged in a TO can.
Another aspect of the invention is a fiber optic transceiver, comprising: a fiber optic interface to receive optical data into the fiber optic and transmit optical data from the fiber optic transceiver; a transmit section comprising a laser and laser driver and safety circuits to generate and transmit optical data from the fiber optic transceiver; a receiver section, further comprising a photodetector to receive the optical data and generate a peak current signal in response to the optical power of the optical data and a transimpedance amplifier having an optical power meter to convert the peak current to a peak voltage signal and a post amplifier to further process the peak voltage signal; and a host interface connected to both the receive and transmit sections to couple electrical signals to the fiber optic transceiver. The post amplifier may extract the optical power from the peak voltage. The peak voltage may be input directly to the post amplifier without affecting the peak voltage signal. Alternatively, the peak voltage may control the peak voltage signal, and the post amplifier may extract the optical power by decoupling the peak voltage signal from the peak voltage. The transimpedance amplifier may further have a voltage signal generating circuit which generates a voltage data signal in response to the optical data; and a current sink which sinks current from the voltage signal generating circuit in response to the common mode voltage so that the voltage data signal is imposed on the common mode voltage.
Another aspect of the invention is an optical power meter, comprising: means to receive an optical signal; means to convert the optical signal to a peak-to-peak current; means to convert the peak-to-peak current to a peak-to-peak voltage; and means to derive a common mode peak control voltage from the peak-to-peak voltage, the common mode peak control voltage in a known relationship with the power of the optical signal. The optical may further comprises a means to drive the peak-to-peak voltage with the common mode control voltage. And yet, the optical power meter of may still further comprise a means to differentiate between the peak-to-peak voltage and the common mode control voltage; and means to determine the optical power from the common mode control voltage.
The invention may still yet be considered a method to measure the optical power of transmitted light, the method comprising the steps of: converting the transmitted light to a peak-to-peak current; converting the current to a voltage signal; detecting the voltage signal to obtain a peak voltage; and determining that the peak voltage is related to the optical power of the transmitted light. The peak voltage is in a known relationship to the optical power of the transmitted light. The method may yet further comprise imposing the voltage signal on the peak voltage.
The invention may still be considered a method of deriving the optical power of transmitted light, comprising: receiving a peak-to-peak voltage signal indicative of data of the transmitted light; decoupling a common mode control voltage from the peak-to-peak voltage signal; and determining the optical power from the common mode control voltage knowing a relationship between the optical power and the common mode control voltage. A linear relationship may exist between the optical power and the common mode control voltage.